1. Field of the Invention
This invention relates to bonding tool tips in general and more particularly to ceramic tool tips for bonding electrical connections.
2. Description of the Prior Art
Integrated circuits are typically attached to a lead frame, and individual leads are connected with wire to individual bond pads on the integrated circuit. The wire is fed through a tubular bonding tool tip having a bonding pad at the output end. These tips are called capillary tips. An electrical discharge at the bonding tool tip supplied by a separate Electronic Flame Off (EFO) device melts a bit of the wire, forming a bonding ball. Other bonding tools do not have the center tube, but have a feed hole or other feature for feeding the wire along, as needed. Some bonding tool tips have no such arrangement for feeding wire, such as bonding tool tips for magnetic disk recording devices, where the wire is insulated and bonded to a magnetic head and then to a flexible wire circuit.
When the bonding tool tip is on the integrated circuit die side of the wire connection, the wire will have a ball formed on the end of the wire, as above, before reaching the next die bonding pad. The ball then contacts the film formed on the die pad on the integrated circuit. The bonding tool tip is then moved from the integrated circuit die pad, feeding out gold wire as the tool is moved, onto the bond pad on the lead frame, and then scrubbed laterally by an ultrasonic transducer. Pressure from the bonding tool tip and the transducer, and capillary action, causes the wire to xe2x80x9cflowxe2x80x9d onto the bonding pad where molecular bonds produce a reliable electrical and mechanical connection.
Bonding tool tips must be sufficiently hard to prevent deformation under pressure, and mechanically durable so that many bonds can be made before replacement. Prior art bonding tool tips were made of aluminum oxide, which is an insulator that is durable enough to form thousands of bonding connections. Bonding tool tips must also be designed to produce a reliable electrical contact, yet prevent electrostatic discharge damage to the part being bonded. Certain prior art devices emit one or more volts when the tip makes bonding contact. This could present a problem, as a one volt static discharge could cause a 20 milliamp current to flow, which, in certain instances, could damage the integrated circuit or magnetic recording head.
U.S. Pat. No. 5,816,472 to Linn describes a durable alumina bonding tool xe2x80x9cwithout electrically conductive metallic bindersxe2x80x9d that is therefore an insulator. U.S. Pat. No. 5,616,257 to Harada describes covering a bonding tool electrode with an insulating cap or covering xe2x80x9cmade of a ceramic materialxe2x80x9d to produce a large electrostatic discharge that creates bonding balls of stable diameter. U.S. Pat. No. 5,280,979 to Poli describes a vacuum wafer-handling tool having a ceramic coating xe2x80x9cmade with a controlled conductivityxe2x80x9d to prevent a large electrostatic discharge.
The present invention may provide electrically dissipative ceramic bonding tool tips for bonding electrical connections to bonding pads on electrical devices. In accordance with principles of the present invention, the method of using the invention involves an added step of dissipating electrical charge at a rate sufficiently high to prevent charge buildup, but not high enough to overload the device being bonded. This added step is at least partially counter-intuitive because ordinarily charge dissipation is avoided so as not to overload the circuit. Consequently, to avoid damaging delicate electronic devices by any electrostatic discharge, the bonding tool tip is made to conduct electricity at a rate sufficiently high to prevent charge buildup, but not high enough to overload the device being bonded. In other words, it is desirable for the bonding tool tip to discharge slowly. The tip needs to discharge to avoid a sudden surge of current that could damage the part being bonded. For best results, a resistance in the tip assembly itself should range from about 5xc3x97104 or 105 to 1012 ohms. This range of resistances is adequate no matter the method of characterizing the resistance. The tools may also have a high stiffness and high abrasion resistance so that the tools have a long lifetime. However, bonding tool tips having a low stiffness and low abrasion resistance may also be made, except that they would have a short lifetime. Possible materials that can be used for the bonding tool tips that have a high abrasion resistance and high stiffness include ceramics (electrical non-conductors) or metals, such as tungsten carbide (an electrical conductor).
In the present invention, bonding tool tips with the desired electrical conduction can be made in at least three different configurations.
First, the tools can be made from a uniform extrinsic semiconducting material that has dopant atoms in the appropriate concentration and valence states to produce sufficient mobile charge carrier densities (unbound electrons or holes) that will result in electrical conduction in the desired range. For example, the tools can be made from polycrystalline silicon carbide uniformly doped with boron.
Second, the tools can be made with a thin layer of a highly doped semiconductor on an insulating core. In this case, the core provides the mechanical stiffness and the semiconductor surface layer provides abrasion resistance and provides a charge carrier path from the tip to the mount that will permit dissipation of electrostatic charge at an acceptable rate. For example, the tools can be made from a diamond tip wedge that has a surface that is ion implanted with boron.
Third, the tools can be made with a lightly doped semiconductor layer on a conducting core. The conducting core provides the mechanical stiffness and the semiconductor layer provides abrasion resistance and provides a charge carrier path from the tip to the conducting core, which is electrically connected to the mount. The doping level is chosen to produce a conductance through the layer that will permit dissipation of electrostatic charge at an acceptable rate. For example, the tools can be made from a cobalt-bonded tungsten carbide coated with titanium nitride carbide.